const arr1 = [1, 2, 3];
const arr2 = [2, 4, 5];
const arr3 = [7, 8, 9];
console.log(arr1);
console.log(arr1.concat(arr2, arr3));

let timerId;

function start() {
  if (timerId) {
    // 当前已经有定时器在运行了
    return;
  }
  timerId = setInterval(() => {
    console.clear();
    console.log(new Date().toLocaleString());
  });
}
function stop() {
  clearInterval(timerId);
  timerId = null;
}
